Sorry, I wasn't ignoring you... got called away. I took a look at the diagram for your unit and it looks like that furnace control is an optional feature. You would likely have been made aware of that when they installed it, assuming you were the initial buyer.
That actually makes it somewhat easier to install a control system that only engages the furnace blower when the HRV is in high mode.
But first consider just unplugging the optional furnace blower control system and never have the blower come on by HRV control. That must be an acceptable mode since that control is an optional feature... implies it will work without it. If that's not acceptable, read on:
You would need to install a current sensing switch on the high speed wire of the HRV unit's motor. Then run a 2 cond. wire from that and cut and connect it in series with one of the control coil feed wires (either one, doesn't matter which) going to the that optional furnace control relay's coil (#1 or #3 terminal). That will stop the relay from energizing until the HRV goes into high speed mode.